R. Ganesh Narayanan

Predicting the forming limit strains of tailor-welded blanks

This work aims at predicting the forming limit strains of welded blanks using a thickness gradient-based necking criterion. In order to reduce the complexity and synergistic effect of tailor-welded blank (TWB) parameters, the same material and thickness sheets are considered for the entire work. The forming limit curve (FLC) of welded blanks for varied weld conditions – namely, weld orientation and location – are predicted by simulating the limit dome height (LDH) test using PAM-STAMP (ESI Group, PAM System International), a finite element code. The un-welded blank and TWB FLCs thus predicted by the thickness gradient-based necking criterion are compared with the experimental FLCs obtained by LDH test. Dome height at failure and failure location data of TWBs from experiments and prediction are also compared for varied weld conditions. It is found from the analyses that limit strain predictions correlate well (less than five per cent error) with the experimental results in the drawing region of the forming limit diagram, with the stretching region showing considerable difference. Dome height at failure and failure location prediction correlate well with those of experimental observation in most of the cases. In a few TWB cases, where considerable difference in limit strains are seen, predicted dome height at failure values deviates from experimental results.

Influence of the weld conditions on the forming-limit strains of tailor-welded blanks

The main objective of the present work is to study experimentally the influence of just the weld conditions, namely the weld region, weld orientation, and weld location, on the forming-limit strains of steel laser-welded blanks. Transverse and longitudinal weld orientations are considered for this study. The weld location includes both centre and offset weld positions in the transverse weld orientation. To investigate the forming behaviour of tailor-welded blanks, limit dome height (LDH) tests are performed. Forming behaviour is quantified by the forming-limit curve (FLC). The FLCs of welded blanks are compared with that of an unwelded blank. It is found from the analyses that the weld region shows a negligible effect on the FLC when placed at the centre position irrespective of weld orientation. In the case of transverse welded blanks, when placed at offset locations (20 mm and 30 mm from geometric centre), limit strains are found to decrease when compared with that of the unwelded blank, with the 30 mm offset location showing the minimum FLC. The offset weld locations (20 mm and 30 mm offset) show reduced limit strains when compared with that of the centre weld location depicting the intrinsic effect of the weld location on the FLC. More variation is seen especially in the stretching region of the forming-limit diagram, while less variation is observed in the drawing region. The FLCs of longitudinal- and transverse-welded blanks with a weld at the centre are nearly identical.

Application of a few necking criteria in predicting the forming limit of unwelded and tailor-welded blanks

The limiting value of strain describing necking/failure during a sheet stamping operation is quantified, predicted by the forming limit curve (FLC). The main aim of the present work is to analyse the applicability of the few existing necking criteria, namely the effective strain rate based criterion (ESRC - RC1), major strain rate based criterion (MSRC - RC2), thickness strain rate based criterion (TSRC - RC3), and thickness gradient based criterion (TGNC - RC4), in predicting the forming limit of unwelded and tailor-welded blanks. In the case of unwelded blanks, dry (m 50.12) and low-friction (m 50.02) conditions are simulated and forming limit results are predicted. In the case of tailor-welded blanks, the forming limits of laser-welded steel blanks and friction-stir-welded blanks made of aluminium alloy are predicted using the chosen necking criteria and then compared with results in the literature. The influence of friction on the whole forming limit curve and the non-linearity of strain paths in the stretching side of the forming limit diagram (FLD) are also predicted and discussed. It is found that the FLCs predicted by ESRC, MSRC, TSRC, and TGNC are comparable with experimental FLCs. The predictions are accurate in the low-friction condition, indicating that the necking criteria are also valid under changing friction conditions. Moreover, the overall FLC does not change much with changing friction conditions. The strain paths are non-linear in the dry friction condition, but they become linear in the low-friction condition. In the case of laser- welded blanks, the original strain rate based criteria defined for unwelded blanks under- estimate limit strains on the stretching side of the FLD, where failure near the weld region is witnessed. Hence they are modified as RC1 >25, RC2 >32, and RC3 >32, which show a better prediction level compared with the original criteria. The original and modified limit strain criteria show better correlation with results in the literature and the TGNC in the case of friction-stir-welded blanks modelled using the Hollomon and Swift strain-hardening laws.The limiting value of strain describing necking/failure during a sheet stamping operation is quantified, predicted by the forming limit curve (FLC). The main aim of the present work is to analyse the applicability of the few existing necking criteria, namely the effective strain rate based criterion (ESRC – RC1), major strain rate based criterion (MSRC – RC2), thickness strain rate based criterion (TSRC – RC3), and thickness gradient based criterion (TGNC – RC4), in predicting the forming limit of unwelded and tailor-welded blanks. In the case of unwelded blanks, dry (m5 0.12) and low-friction (m5 0.02) conditions are simulated and forming limit results are predicted. In the case of tailor-welded blanks, the forming limits of laser-welded steel blanks and friction-stir-welded blanks made of aluminium alloy are predicted using the chosen necking criteria and then compared with results in the literature. The influence of friction on the whole forming limit curve and the non-linearity of strain paths in the stretching side of the forming limit diagram (FLD) are also predicted and discussed. It is found that the FLCs predicted by ESRC, MSRC, TSRC, and TGNC are comparable with experimental FLCs. The predictions are accurate in the low-friction condition, indicating that the necking criteria are also valid under changing friction conditions. Moreover, the overall FLC does not change much with changing friction conditions. The strain paths are non-linear in the dry friction condition, but they become linear in the low-friction condition. In the case of laserwelded blanks, the original strain rate based criteria defined for unwelded blanks underestimate limit strains on the stretching side of the FLD, where failure near the weld region is witnessed. Hence they are modified as RC1> 25, RC2> 32, and RC3> 32, which show a better prediction level compared with the original criteria. The original and modified limit strain criteria show better correlation with results in the literature and the TGNC in the case of friction-stir-welded blanks modelled using the Hollomon and Swift strain-hardening laws.

Investigation on the influence of adhesive properties on the formability of adhesive-bonded steel sheets

The main aim of this research work is to study the influence of adhesive properties on the formability of adhesive-bonded steel sheets. The adhesive properties were varied by having two different adhesives, epoxy based and acrylic based, and by changing the hardener to resin ratios. The deep drawing quality cold rolled steel and stainless steel (SS 316L) sheets were used as base materials. The epoxy and acrylic adhesives show improved elongation with increase in hardener to resin ratio. This is because of changeover of resin-rich formulation to hardener-rich formulation, making the sample more ductile. The adhesive-bonded blanks show improved elongation as compared to double sheets, which is due to the presence of adhesive delaying the onset of necking. With increase in hardener to resin ratio of both the adhesives, the elongation of individual sheets has improved. This is due to the improvement in elongation of adhesives with increase in hardener to resin ratio. The strain hardening exponent (n) of adhesive-bonded blanks has improved with increase in hardener to resin ratio in all the regions of deformation. The limit strain of deep drawing quality and SS 316L sheets constituting adhesive-bonded blanks shows improvement with increase in hardener to resin ratio. The adhesive-bonded blanks with interface bonding exhibit better limit strain as compared to the case without interface bonding.

Influence of Friction in Simple Upsetting and Prediction of Hardness Distribution in a Cold Forged Product

Predicting inhomogeneous deformation in any forging process will definitely be helpful in deciding the tool, billet material, lubrication, annealing sequences, and number of stages to make products. In this work, the influence of varied friction conditions on the hardness and effective strain variation during simple upsetting is studied. Also, hardness variation in a typical cold forging process is predicted by relating hardness and effective strain evolution in a simple upsetting operation empirically. Four different lubricants, viz., castor oil (m=0.33), soap (m=0.25), grease (m=0.2), teflon (m=0.16), are considered for experimentation. The friction factors of these lubricants were obtained from a Ring Compression Test (RCT) and are used in FE simulations of upsetting and forging operations. It is found from the analyses that: (1) Teflon shows relatively less variation in hardness and effective strain depicting homogeneous upsetting operation, whereas other lubricants show a larger variation in hardness and effective strain in radial and axial directions; (2) hardness is observed to vary linearly with effective strain; (3) the empirical relationship between hardness and effective strain obtained from a simple upsetting operation, which is common for all the lubricants, predicts the hardness distribution during the forging-extrusion process with moderate accuracy. This depends on the interface friction conditions, i.e., solid and semi-solid lubricants with better holdability like Teflon and soap show good correlation between experimental and predicted hardness values than liquid lubricant, i.e., castor oil.

Assessing the Validity of Original and Modified Failure Criteria to Predict the Forming Limit of Unwelded and Tailor Welded Blanks with Longitudinal Weld

The main objective of the work is to assess the validity of modified failure criteria proposed by Naik et al. [1] and original failure criteria in predicting the forming limit of unwelded and tailor welded blanks (TWBs) with longitudinal weld, during stretching and drawing operations. The four different failure criteria, namely, effective strain rate-based, major strain rate-based, thickness strain rate-based, and thickness gradient-based failure criteria, both in original and modified forms, are used to predict the forming limit strains of unwelded and welded blanks. In the case of unwelded sheet, the forming limit predictions are consistent with the experimental result in the drawing side, but considerable difference is observed in the stretching side of forming limit diagram (FLD). In the case of tailor welded blanks (TWB), whenever failure is seen near the weld region, the strain rate-based criteria are modified as RC1 ≥ 25, RC2 ≥ 32, RC3 ≥ 32 for failure to occur. TWB forming limit curve (FLC) predicted using the modified criteria show better accuracy than the original failure criteria in the stretching side of the FLD. In the drawing side, FLC predicted by original criteria is the same as that from modified criteria, where in strain path change is witnessed instead of limit strain improvement. Through this work, it is demonstrated that using modified failure criteria, forming limit of TWBs can be predicted with better accuracy during stretching operation, while original failure criteria are sufficient for a drawing operation or in the drawing side of FLD

CAFE modeling, neural network modeling, and experimental investigation of friction stir welding

The main aim of the present work is to develop a cellular automata finite element –artificial neural network (CAFE-ANN) hybrid model to predict the evolution of grain size and yield strength during friction stir welding. The CAFE model was developed by linking CA cells with elements in ABAQUS, a finite element code. Then a neural network was developed and trained appropriately by using the data obtained from the validated CAFE model that predicts the grain size and yield strength. The ANN results are validated. Finally it has been demonstrated that ANN can be used as a ‘virtual machine’ to examine the effect of rotational speed, welding speed, axial force and shoulder diameter on the variation in grain size and yield strength. The temperature and strain-rate distribution predicted by the thermal and strain-rate models are consistent with the existing results. The CAFE model grain size predictions are also accurate as compared to experiments. The grain size and yield strength predictions from ANN coincide well with the experimental values and CAFE model predictions. It has been demonstrated that CAFE-ANN hybrid model can be used as a ‘virtual machine’ to predict and analyze the effect of above said parameters on the grain size and yield strength evolution. The predicted influence is found to agree with some of the available results. In few cases, a slight deviation in the trend is witnessed as compared to literature results.

Influence of tool rotation speed and feed rate on the forming limit of friction stir welded AA6061-T6 sheets

In this study, the influence of tool rotation speed and feed rate on the forming limit of friction stir welded Al 6061-T651 sheets has been investigated. The forming limit curve was evaluated by limit dome height test performed on all the friction stir welded sheets. The welding trials were conducted at a tool rotation speed of 1300 and 1400 r/min and feed rate of 90 and 100 mm/min. A third trial of welding was performed at a rotational speed of 1500 r/min and feed rate 120 mm/min. It is found that with increase in the tool rotation speed, from 1300 to 1400 r/min, for a constant feed rate, the forming limit of friction stir welded blank has improved and with increase in feed rate, from 90 to 100 mm/min, for a constant tool rotation speed, it has decreased. The forming limit of friction stir welded sheets is better than unwelded sheets. The thickness gradient after forming is severe in the cases of friction stir welded blanks made at higher feed rate and lower rotation speed. The strain hardening exponent of weld (n) increases with increase in tool rotation speed and it decreases with increase in feed rate. It has been demonstrated that the change in the forming limit of friction stir welded sheets with respect to welding parameters is due to the thickness distribution severity and strain hardening exponent of the weld region during forming. There is not much variation in the dome height among the friction stir welded sheets tested. When compared with unwelded sheets, dome height of friction stir welded sheets is higher in near-plane-strain condition, but it is lesser in stretching strain paths.

Influence of external weld flash on the in-plane plane-strain formability of friction stir welded sheets

The main aim of the present work is to analyze the influence of external weld flash on the formability of friction stir welding sheets through in-plane plane-strain formability tests. The load-extension behavior and forming limit strains are measured to quantify the formability. The influence of friction stir welding parameters on the height of weld flash was also studied. The base materials used for welding trials are AA6061T6 and AA5052H32 alloy sheets of 2.1-mm thickness. It is observed that the influence of external weld flash on the maximum load and total extension for all the friction stir welding conditions is negligible. The effect of weld flash on the limiting major strain is also insignificant. But the presence of weld flash has changed the limiting minor strain, more toward plane-strain condition, indicating the change in strain-path toward plane-strain. This is due to the strain taken by weld flash, along with the major strain, minor strain, and thickness strain in the friction stir welding sheet plane because of constancy of volume. The formation of weld flash and its height are affected synergistically by the axial force and temperature development during friction stir welding. The higher the axial force and temperature, the higher the flash height.

Sustainable and green manufacturing and materials design through computations

The main aim of the review is to address the importance of computations in the analyses of manufacturing processes during efficient materials design, tool design, and process design. Initially, the evolution of computations in manufacturing industries is highlighted. The applications of different computational approaches like numerical simulations, analytical modeling, soft computing, etc. in solving metal forming and joining problems are presented. Though there are several manufacturing processes, metal forming and joining applications are included in the present work. The later part of the review is about green manufacturing and sustainability, their relation with computations and manufacturing education. The green manufacturing and sustainability practices in different industries/fields (like automotive, machining, steel, food processing, computers, electronics) are tabulated. The significance of computations in green manufacturing and sustainability issues are described with a few examples from available literature. The last part of the work is about improving the awareness of green and sustainability concepts through manufacturing education. In this, the possible ways of modifying the present course content of ‘manufacturing technology’ in under-graduate education are suggested, by giving importance to green and sustainability concepts. The role of computations in manufacturing education is summarized finally.